Acrylic pressure sensitive adhesive compositions have been widely used in industry for the production of industrial sheet, optical pressure sensitive adhesive product, and pressure sensitive adhesive for electrical components, etc, and are applied to other industrial fields with the same conception.
Among many products, liquid crystal display (LCD) is taking the place of CRT to be used in various high-tech fields producing notebook computer, monitor, TV, etc, owing to its potential for weight reduction and slimness.
In general, to produce LCD, liquid crystal cell and polarizer are basically required, and for the adhesion of the two, proper pressure sensitive adhesive layer should be applied. Polarizer includes multilayer such as a row of iodine compounds or dichroic polarizing material, protective film, such as triacetyl cellulose (TAC), to protect polarizing element on both sides, pressure sensitive adhesive layer, and releasing film, etc.
The production of LCD includes the process of adhering polarizer, during which static electricity is generated after peeling releasing film out of pressure sensitive adhesive layer. The generated static electricity affects orientation of liquid crystal in LCD, resulting in defectives.
Generation of static electricity can be inhibited by forming antistatic layer outside polarizer, whose effect is, though, too low to prevent the fundamental generation of static electricity. Therefore, in order to prevent the generation of static electricity in the first place, it is important for a pressure sensitive adhesive to have an antistatic property.
To solve the problem of static electricity, various attempts have been made such as adding metal powders having conductivity or conductive components such as carbon particles to pressure sensitive adhesive, adding ionic substances like surfactants to pressure sensitive adhesive, etc. However, in order for conductive components or carbon particles to prevent the generation of static electricity, a large amount of them has to be added, dropping the transparency. And the addition of surfactants to pressure sensitive adhesive results in the decrease of the pressure sensitive adhesive property by transitivity to the surface of the pressure sensitive adhesive and the product vulnerable for humidity.
An attempt has been made to prevent the generation of static electricity by making a pressure sensitive adhesive flexible by adding ethylene oxide denatured phthalic acid dioctyl plasticizer to the pressure sensitive adhesive (Japanese Patent Publication No. 5-140519). However, the addition of such plasticizer only cannot prevent the fundamental generation of static electricity and not to eliminate static electricity remaining after the separation of releasing film, either.
In Japanese Patent Publication No. 6-128539, it is described that static electricity is prevented by mixing polyether polyol compound and at least one of alkali metallic salt to an acrylic pressure sensitive adhesive. But, when polyether polyol compound is added to a pressure sensitive adhesive having isocyanate crosslinking agent, the crosslinking degree is affected greatly. To avoid of such affection on the crosslinking degree, the inventors of the above patent tried to make crosslinking first by using isocyanate crosslinking agent, and then dissolved the composition again to mix polyether polyol compound and added alkali metallic salt thereto. However, such processes are very difficult to be applied in real industry.
Therefore, it is required to develop a pressure sensitive adhesive having excellent antistatic property without changing of compatibility and durability.
Thus, the present inventors tried to solve the above mentioned problems, and found that the generation of static electricity can be satisfactorily inhibited when ester plasticizer having excellent compatibility and at least one of ether bonds in its molecule and alkali metallic salt are added together to a pressure sensitive adhesive. And, the present inventors completed this invention by producing an acrylic pressure sensitive adhesive composition having antistatic property without damaging durability by adding the above ester plasticizer and alkali metallic salt together to an acrylic pressure sensitive adhesive.